Abrasive sectors and mounting apparatus



1964 R. e. VAN KEUREN ETAL 3,117,603

ABRASIVE SECTORS AND MOUNTING APPARATUS Filed Aug. 25, 1961 s Sheets-Sheet 1 I m 65 2 7E I LLJ INVENTORS ROBERT G. VAN KEUEEN ATTO RNE'Y 14, 1964 R. e. VAN KEUREN ETAL 3,117,603

ABRASIVE SECTORS AND MOUNTING APPARATUS Filed Aug. 25, 1961 3 Sheets-Sheet 2 INVENTORS ROBERT 6. WW KEUEEN BY THE/ea 601.5

Jan- 14, 1964 R. G. VAN KEUREN ETAL ABRASIVE SECTORS AND MOUNTING APPARATUS 3 Sheets-Sheet 3 Filed Aug. 25, 1961 E0 5 E ix r/ $385M BEET A/Z COLE 7 EZON H BY A TORNEY United States Patent 3,117,603 ABRASIVE SECTORS AND MOUNTING AIPARATUS Robert G. Van Keuren and Theron M. Cole, Holden, Mass, assignors to Norton Company, Worcester, Mass,

a corporation of Massachusetts Filed Aug. 25, 1961, Ser. No. 133,963 8 Claims. (Cl. 146-182) The invention relates to abrasive sectors and mounting apparatus for pulp refiners, and other grinders.

This application is a continuation-in-part of application S.N. 804,667 to R. G. Van Keuren, filed April 7, 1959, now abandoned.

One object of the invention is to provide sector constructions which will infrequently fail in use. Another object is to provide sector constructions which will Withstand severe operating conditions, over 12,000 surface feet per minute speed with very high driving torque. Another object is to provide sectors and a mounting therefor to withstand high centrifugal force. Another object is to provide a mounting for sectors designed to distribute the driving torque. Another object is to provide a clamping bar which clamps the abrasive sectors in compression against the backing plate and which also acts as a chip breaker. Another object of the invention is to provide sectors useful for grinding food products, such as peanuts to make peanut butter together with clamping apparatus therefor in accordance with the preceding objects.

Other objects will be in part obvious or in part pointed out hereinafter.

In the accompanying drawin s illustrating a pulp refiner, backing plates, sectors and mountings therefor,

FIGURE 1 is a view partly in elevation and partly in vertical section of a typical Bauer pulp refiner machine and the backing plates with the sectors mounted therein,

FIGURE 2 is an elevation of the sectors of FIGURE 1 on a backing plate,

FIGURE 3 is a cross-sectional view on the line 33 of FIGURE 2,

FIGURE 4 is a cross-sectional view on the line 44 of FIGURE 2,

FIGURE 5 is a sectional view similar to FIGURE 4 showing another embodiment of the invention,

FIGURE 6 is an elevation of a modified mounting of a sector on a backing plate for use in a Sprout-Waldron machine, and

FIGURE 7 is a View taken on line 77 of FIGURE 6.

For pulp refiners, abrasive sectors, sometimes called segments, have certain advantages over metal plates and metal sectors, but due to the brittleness of bonded abrasive material they have in general had short life. Sometimes they have broken within a few hours after being put in operation. However, as compared with metal plates, etc., they show very desirable operating characteristics; improved pulp qualities, lower power requirements and increased machine capacity.

The construction herein described can be operated for long periods of time with no indication that breakage is imminent. Hence these abrasive sectors and mounting apparatus are satisfactory from a mechanical point of view, something which had not been previously achieved, and they still give the expected advantages in operating characteristics above stated. We will briefly describe first one well known design of pulp refiner, namely, the Bauer machine, to show the use of our invention.

Referring to FIGURE 1, the pulp refiner has a base 11 which is hollow and through which the refined pulp goes out. This base 11 supports an annular frame 12 the bottom of Which is open and the top and sides of which are closed. The annular frame 12 has a wide circular opening 13 on the left hand side and another one 14 3,117,603 Patented Jan. 14, 1964 on the right hand side, the latter being covered with a plate 15. Closing the opening 13' is a connecting portion 16 of a hopper 17 to which pulp to be refined is introduced into the inside of the annular frame 12.

Mounted on the left hand side of the base 11 is an electric motor 21 having a shaft 22 which extends through a hole in the connecting portion 16. Mounted upon the right hand side of the base 11 is an electric motor 23 having a shaft 24 which extends through a hole in the plate 15. These shafts 22 and 24 revolve in opposite directions and at about the same speed. Mounted on the shaft 22 and keyed to it is a circular plate 25. The circular plate 25 has holes 27 through it for admission of pulp between the backing plates 25 and 26. The backing plate 25 likewise has a sealing ring 23 on an annular projection 29 running in a slot 30 in the connecting portion 16 and overlapping the edge of the wide circular opening 13. This description of a refiner is brief because it is no part of the present invention and merely shows the application of the abrasive sectors and mounting apparatus.

Still referring to FIGURE 1, bolts 31 secure backing plates 32 to the circular plates 25 and 26. In this embodiment of the invention there is one backing plate 32 secured to each circular plate 25 and 26.

Referring now to FIGURE 4, sector shaped steel wedges 35 are secured by bolts 36 to the backing plates 32. In this embodiment of the invention there are six steel wedges 35 secured to each backing plate 3 2 and the wedges 35 are in contiguous relation. The backing plates 32 have circular lips 37 at the periphery. Abrasive sectors 41 are secured between the circular lips 37 and the steel wedges 35. This is a feature of especial importance in the present invention and eliminates the use of inserted lugs in the abrasive sectors which have given trouble. The fit of these parts is such that the bolts 36 can draw the wedges 35 to press the sectors 41 against the lip 37 thus holding the sectors tightly in place. Additionally cement 42 is used to hold the sectors in place.

Referring to FIGURES 2 and 3, steel keys 45 are secured by screws 46 to the backing plates 32. These keys 45 fit in cut-outs '47 formed radially in the under edges of the sectors 41. These keys 45 drive the sectors 41 without strain.

Radial grooves 51 are formed in the sectors 41 to guide the pulp to the plane surfaces 52 where it is ground. Radial grooves 53 are formed in the surfaces of the wedges 35 and the grooves 53 are matched with the grooves 51. Thus continuous channels for the pulp between the circular plates 25 and 26 are provided. As the pulp moves in through the hopper 17, it moves to the space between the plates 25 and 26 and is driven between the sectors 41 on the right and the sectors 41 on the left. It appears to be ground somewhat before it reaches the plane surfaces 52. The clearance between can be adjusted by a machine adjustment well known and not shown herein. This adjustment moves one plate 25 or 26 relative to the other one. The usual clearance between the surfaces 52 is a matter of from a fraction of a thousandth to several thousandths of an inch and of course the smaller the clearance the finer will be the pulp. It will be remembered that the plates 25 and 26 are rotating in opposite directions and nevertheless the pulp is ejected by some force we do not understand since with one set of sectors moving in one direction and the other set of sectors moving in the other direction, the pulp cannot be rotating very fast. However, probably the initial rotation by the plate 25 prevails.

Heretofore in actual use, except experimental, sectors have been made out of steel. Abrasive sectors have been previously proposed and tried but breakage has been prohibitive. Mostly vitrified bonded abrasive sectors were tried. In an effort to eliminate breakage, phenolic resin bonded sectors have been tried, but these rapidly deteriorated from the hot water and acid therein. Rubber bonded sectors have stood up pretty well but they dont grind the pulp as fast. For a satisfactory pulp, vitrified bonded abrasive sectors are best and they are not deteriorated by the hot liquor. But heretofore they have soon been broken in operation. Sectors as herein described mounted as herein described made out of abrasive vitrified bonded can be successfully used for long periods without failure. They are producing a better grade of pulp than the steel sectors which they have replaced.

One set of sectors in operation was made out of 46 grit size regular aluminum oxide grain, bonded with a common grade of vitrified bond of 54 volume percent of abrasive and 12.4 volume percent of bond, leaving 33.6 volume percent of pores. The other set was made out of 16 grit size white aluminum oxide abrasive bonded with a common vitrified bond, of 54 volume percent of abrasive and 16 volume percent of bond leaving volume percent of pores.

We cannot choose between these because one was for one kind of pulp and the other was for another kind of pulp and they are both doing equally well. However, for purposes for complying with the statute, we select the first of these as the best mode. We believe that aluminum oxide is the best abrasive for the particular pulps involved but also believe that for certain pulps, silicon carbide will be preferred. Very likely for still other kinds of pulps, zirconia abrasive will be preferred. As other abrasives might be used, we should not be limited thereto. As vitrified bond has previously been proposed for sectors for refiners and as very likely at some future time a satisfactory organic bond will be found, we should not be limited to the particular bond used. Although the grooving of metal sectors is standard practice and the groves have been of many different shapes and sizes and orientations, continuous grooves from the inside to the plane surfaces 52 are best, so we can claim them for definition of the sectors per se. In the general combination grooving need not always be claimed since probably for some kinds of pulp grooving might be unnecessary.

While the invention is unliim'ted so far as size is concerned, to complete the illustrative embodiment, the outside diameter of the sectors 41 in place, which is the same as the outside diameter of the backing plates 32, is 36 inches, the other dimensions in proportion as shown.

The wedges are clamping bars and collectively in each unit constitute a clamping bar. As stated in the objects they act as chip breakers. They break up any large pieces of pulp that find their way into the refiner and as they are stronger than the bonded abrasive sectors 41, they protect the latter from damage. In this respect their grooves 53 are important. Sometimes knots enter the refiner.

' The clamping bars 35, otherwise called wedges, when i drawn by the screws 36, force the sectors 41 against the lips 37. This produces a compression of the sectors 41 against the backing plates 32 and of course any material is stronger in compression than in tension, at least this is true of bonded abrasive material. Furthermore the strains are mostly circular, and holding the sectors by radial compression (as well as by horizontal compression) which the wedges 35 do, gives little chance for breakage since if they do slip a little they still will not be broken whereas the inserted lugs could not move at all thus breaking the sectors. With this wedge clamping means, the driving keys also cooperate because they back up the circular'resultant thrust on the sectors and drive them with- V out strain. They foreclose any serious amount of sli page.

4 strain. It is resistant to the liquor in the pulp being refined.

For the best results the epoxy resin cement should be mixed with a mineral filler. Preferably this should be fine. We have used and found satisfactory aluminum oxide dust collector fines. This is almost waste material and therefore inexpensive. A proportion of 20% by volume of the aluminum oxide dust collector fines of the cement and fines is the best proportion known to us. Any other mineral filler of the right size could be used.

it is possible to vary the construction in many ways although the construction described and illustrated in FIGURES 14 is the best one now known to us. A nearly equally good embodiment is disclosed in FIGURE 5. The circular plate 26 has attached thereto by means of bolts 61 a backing plate 62. This has an integral clamping bar 65 extending all the Way around it. Bolts 66 secure wedges 67 to the backing plate 62 and in this case also there are six steel wedges 67 secured to each backing plate '62 and the wedges 67 are in contiguous relation. Abrasive sectors 71 are secured between the integral clamping bars 65 and the wedges 67. The same cement 72 can be used to assist in holding the sectors in place.

Radial grooves 81 are formed in the sectors 71 for the same purpose as the radial grooves 51. Also radial grooves 83 are formed in the surfaces of the integral clamping bars 65 for the same purpose as the radial grooves 53. The same steel keys 45 can be used to drive the sectors without strain.

in the first embodiment the wedges 35 are inclined and the circular lips 37 are straight. In the second embodiment the integral clamping bars 65 are inclined and so are the wedges 67. In both embodiments both could be inclined but it is a little more difficult to shape the sectors on the outside to match. On the inside, the taper shown is easy to form in the sectors.

The only disadvantages of the embodiment of FIGURE 5 that we can see are the slightly greater difiiculty of shaping the sectors 71 mentioned and the fact that the bolts 66 must hold the centrifugal force of the sectors 71. However, it is believed that they can do it if enough of them are provided and ten bolts 66 can easily be provided in each wedge 67 when there are six sectors per circular plate as shown. As off-setting these possible disadvantages ther may be some advantages in the embodiment of FIGURE 5 which we do not now see.

The construction described so far is that adapted for use in a Bauer machine while the construction shown in FIGS. 6 and 7 is designed for use in a Sprout-Waldron refiner. In this machine one plate is held stationary and the other is driven. The stationary wheel is mounted within a hinged casing member and the machines may be opened up for the replacement of worn segments. With this design of machine the backing plate structure shown in FIGS. 6 and 7 may be used. As here shown a relatively thinner backing plate 93 may be provided for each sector having an integral clamping bar 65 with radial grooves 83. The grooves 83 coact with grooves 91 in the abrasive sectors 92. The sector has an outer peripheral fiat surface 52 to which the pulp fiows as it is worked outwardly from the center, all as described above. The abrasive sector 92, however, is designed to be bolted by headed bolts to a circular plate 96. One plate 96 is fixed and another plate as is supported at the end of a shaft such as driven shaft 24.

The abrasive sector 92 is fitted under clamping bar765 and is adhesively bonded to the thin backing plate 93. The sector is provided with a circular periphery to fit neatly within flange or lip 94 integral with circular drive plate 96 and after the sector has been assembled on the backing plate it may be mounted on the circular plate with the sector 92 firmly adhered to the backing plate. Also the sector is adapted to be tightly engaged between the clamping bar 65 and flange 94 by means of the structure here shown since a suitable adjustment may be made at the time of mounting the sector on the circular plate due to the enlarged holes for receiving bolts 95. In the Sprout-Waldron machine, which is well known in the pulp refining industry, the casing may be swung open and the assembled sector and backing plates may be easily installed with such an assured tight fit that the function of the lip 37 and adjustable clamping bar 35 of FIG. 4 and the clamping bar 65 and wedges 67 of FIG. 5 may be incorporated in the clamping bar 65 and ring 94 of FIG. 7. In all other respects the structure shown in FIGS. 6 and 7 operates in the manner described above.

It is apparent that the fixed flange on the driving plate 96 might be made to cooperate with the inner periphery of the abrasive sector and the wedge means with the outer edge. We prefer, however, to take advantage of the centrifugal force present as the refiner machine is driven, to hold the sector against the fixed flange which engages the outer periphery of the sector.

So strongly do the clamping bars or wedges and the lip, or the clamping bars and wedges, or the clamping bar and flange together with the cement hold the sectors that it has now been found to be possible to dispense with the driving keys 45 at least in connection with that form of the construction shown in FIGS. 6 and 7. As heretofore all abrasive sectors had broken in use we thought the keys were necessary still further to reduce the strain.

In all the embodiments of the invention there are Wedges holding the sectors against clamping means when they are mounted on the driving plates in each of the well known forms of refiners here described. To save difficulty between the singular and the plural, we shall claim a unit combination involving one abrasive sector, one wedge means and one clamping or relatively fixed means.

Further to show the coaction between the clamping or fixed means, or bar and wedge, and the driving keys, if the former were not provided but reliance were had upon the cement alone with merely a lip such as the lip 37 to hold the sectors against centrifugal force, the sectors could move away from the backing plates by pivoting on the driving keys. That could not occur if the driving keys were as thick axially as the sectors, but they cannot be made that way since, by wear of the abrasive, they would be uncovered or, if originally flush, would eventually project beyond the abrasive, and then they would interfere with the grinding of the pulp.

The abrasive sectors are truncated sectors as a perfect sector extends to the center of the circle. They are shown as truncated by circles concentric to their peripheries but, especially if there are many of them, they could be truncated by straight lines or otherwise. They have plane surfaces 52 and plane surfaces where they contact the backing plates. They have broad and wide apex angle frusto-conical surfaces opposite the surfaces that contact the backing plates and these are grooved with continuous grooves from the truncations to the narrow plane surfaces 52. They are thus defined as truncated sectors of bonded abrasive material having plane surface backs, plane circular surfaces opposite the backs adjacent their peripheries merging into wide apex angle frustoconical sectoral surfaces having continuous grooves from their inner edges where they are truncated to said plane circular surfaces, said sectors being thinner from the frusto-conical surfaces to the back where they are truncated than from said plane circular surfaces to the backs.

In operation the peripheries of the sectors shown in FIGS. 1 to 5 have been operated at 12,000 surface feet per minute which required motors 21 and 23 of 300 horsepower apiece. In the future it is probable that they will be operated at even greater speeds because in pulp refiners speeds have been increased through the years. We

elieve the construction herein described has an ample factor of safety to withstand such higher operating speeds. At 12,000 surface feet per minute at the peripheries of the sectors, the shafts 22 and 24 are revolving at 1274 r.p.m.

The same machines have been used for grinding food products such as peanuts to make peanut butter. We believe that abrasive sectors will be far superior in these machines than the present steel plates. In the case of grinding peanuts with abrasive sectors, grooves would probably be quite unnecessary and they would also very likely be unnecessary in the clamping bars or wedges. Even the cement could probably be dispensed with and if the sectors and the backing plates are made very true, the cement can probably be omitted in some pulp refiners, but the cement has a definite advantage in that it equalizes inequalities.

The wood pulp which pulp refiners refine is not always all of it strictly pulp, that is to say some of the wood in the total pulp is frequently in the form of large pieces such as splinters. Usually the product of the grinders is put through two screens before going to the Fourdrinier machine. It is what will not pass these screens that is sent to a refiner and that includes splinters and pulp that is quite coarse. Therefore, by no means is all of the grinding done by the plane circular surfaces 52.

It will thus be seen that there has been provided by this invention abrasive sectors and mounting apparatus in accordance with which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. A sector and mounting apparatus therefor including a circular driving plate for receiving said mounting, comprising: a truncated bonded abrasive sector having an inner and an outer peripheral surface, a generally plane surface back and opposite said back a surface which is grooved to provide a relieved surface merging with a generally circular plane surface, the sector being thickest at said circular plane surface, said grooves being continuous from the inner peripheral surface of the sector to said circular plane surface, a backing plate for said sector adapted to be fixedly secured to said circular driving plate, a wedge means engaging one peripheral edge of said sector, a rigid means for engaging the other peripheral edge of said sector, said Wedge means and rigid means cooperating when said sector is mounted on said circular plate to hold the sector tightly against its backing plate, said means for engaging the inner periphery of the sector for cooperating with the sector having an inwardly extending surface portion forming a lip, said lip being grooved with a plurality of grooves which are continuous with the grooves in the sector and which are continuous across the face of the lip whereby said lip acts as a chip breaker.

2. A sector and mounting apparatus therefor including a circular driving plate for receiving said mounting, comprising: a truncated bonded abrasive sector having an inner and an outer pheripheral surface, a generally plane surface back and opposite said back a generally parallelly disposed surface which is grooved to provide a relieved surface merging with a generally circular plane surface, the sector being thickest at said circular plane surface, said grooves being continuous from the inner peripheral surface of the sector to said circular plane surface, a backing plate for said sector adapted to be fixedly secured to said circular driving plate, a layer of adhesive between said sector and said plate, a wedge means engaging one peripheral edge of said sector, a rigid means for engaging the other peripheral edge of said sector, said wedge means and rigid means cooperating with said adhesive when said sector is mounted on said circular plate to hold the sector tightly against its backing plate, said means for engaging the inner periphery of the sector for cooperating with the sector having an inwardly extending surface portion forming a lip, said lip being grooved with a plurality of grooves which are continuous with the grooves in the sector and which are continuous across the face of the lip whereby said lip acts as a chip breaker.

3. A sector and mounting apparatus therefor including a circular driving plate for receiving said mounting, comprising: a truncated bonded abrasive sector having an inner and an outer peripheral surface, a generally plane surface back and opposite said back a surface which is grooved to provide a relieved surface merging with a generally circular plane surface, the sector being thickest at said circular plane surface, said grooves being continuous from the inner peripheral surface of the sector to said circular plane surface, a backing plate for said sector adapted to be fixedly secured to said circular driving plate, a wedge means engaging the inner peripheral edge of said sector, a rigid flange on said driving plate for engaging the outer peripheral edge of said sector, said wedge means and flange cooperating when said sector is mounted on said circular plate to hold the sector tightly against its backing plate, said wedge means including a surface forming a lip adjacent the inner periphery of the sector for cooperating with the sector, said lip being grooved with a plurality of grooves which are continuos with the grooves in the sector and which are continuous across the face of the lip whereby said lip acts as a chip breaker.

4. A sector and mounting apparatus therefore including a circular driving plate for receiving said mounting, comprising: a truncated bonded abrasive sector having an inner and an outer peripheral surface, a generally plane surface back and opposite said back a surface which is grooved to provide a relieved surface merging with a generally circular plane surface, the sector being thickest at said circular plane surface, said grooves being con: tinuous from the inner peripheral surface of the sector to said circular plane surface, a backing plate for said sector adapted to be fixedly secured to said circular driving plate, a wedge means engaging one peripheral edge of said sector, a rigid means for engaging the other peripheral edge of said sector, said wedge means and rigid means cooperating when said sector is mounted on said circular plate to hold the sector tightly against its backing plate, a lip means adjacent the inner periphery of the sector for cooperating with tr e sector, said lip being grooved with a plurality of grooves which are continuous and 'communicate with the grooves in the sector whereby said lip acts as a chip breaker.

5. A sector and mounting apparatus therefor including a circular driving plate for receiving said mounting, comprising: a truncated bonded abrasive sector having an inner and an outer peripheral surface, a generally plane surface back and opposite said back a surface which is a wide apex angle frusto-conical sectoral surface merging with a circular plane surface, said opposite face being grooved to provide a relieved surface merging with said generally circular plane surface, the sector being thickest at said circular plane surface, said grooves being continuous from the inner peripheral surface of the sector to said circular plane surface, a backing plate for said sector adapted to be fixedly secured to said circular driving plate, a clamping lip for engaging the outer periphery of the sector, said lip being fixedly positioned relative to said backing plate and said driving plate when said sector is secured to said mounting apparatus, a rigid wedge means for engaging the inner peripheral edge of said sector, said wedge means and clamping lip cooperating when said sector is mounted on said circular plate to hold the sector tightly against its backing plate, said wedge means having a breaker lip thereon forming an extension of the inner periphery of the sector and for cooperating With the sector, said breaker lip being grooved with a plurality of grooves which are continuous with the grooves in the sector and which are continuous across the face of the breaker lip whereby to act as a chip breaker.

6. A sector and mounting apparatus therefor comprising: a truncated bonded abrasive sector having an inner and an outer peripheral surface, a plane surface back and opposite said back a surface which is a wide apex angle frusto-conical sectoral surface merging with a circular plane surface, the sector being thickest at the circular plane surface and being grooved with a plurality of continuous grooves from its inner peripheral surface to its circular plane surface; a backing plate for said sector; a clamping lip integral with the outer periphery of the backing plate and engaging the outer periphery of the sector; a wedge means having an exposed face and another surface for a third surface adapted to be engaging the inner periphery of the sector and removably attached to the backing plate, the exposed surface of said wedge means being grooved with a plurality of grooves which are continuous with the grooves in the sector and which are continuous across the surface of the wedge means whereby said wedge means functions as a chip breaker.

7. A sector and apparatus as in claim 6 further comprising: a radial key on the backing plate; and a complementary groove for said key along one of the radial edges of the sector.

8. A sector and mounting apparatus therefor comprising: a truncated bonded abrasive sector having an inner and an outer peripheral surface, a plane surface back and opposite said back a surface which is a wide apex angle frusto-conical sectoral surface merging with a circular plane surface, the sector being thickest at the circular plane surface and being grooved with a plurality of continuous grooves from its inner peripheral surface to its circular plane surface; a backing plate for said sector; a wedge means engaging the outer periphery of the sector and removably attached to the backing plate for clamping the sector to the backing plate; a clamping means integral with the inner periphery of the backing plate and engaging the inner periphery of the sector, said clamping means having a lip portion, said lip being grooved with a plurality of grooves which are continuous with the grooves in the sector and which are continuous across the surface of the clamping lip whereby said clamping lip functions as a chip breaker.

References Cited in the file of this patent UNITED STATES PATENTS 755,989 Baker et a1 Mar. 29, 1904 1,593,153 Ambler July 20, 1926 1,705,996 Pope Mar. 19, 1929 1,795,603 Hussey Mar. 10, 1931 1,814,587 Daniels July 14, 1931 

1. A SECTOR AND MOUNTING APPARATUS THEREFOR INCLUDING A CIRCULAR DRIVING PLATE FOR RECEIVING SAID MOUNTING, COMPRISING: A TRUNCATED BONDED ABRASIVE SECTOR HAVING AN INNER AND AN OUTER PERIPHERAL SURFACE, A GENERALLY PLANE SURFACE BACK AND OPPOSITE SAID BACK A SURFACE WHICH IS GROOVED TO PROVIDE A RELIEVED SURFACE MERGING WITH A GENERALLY CIRCULAR PLANE SURFACE, THE SECTOR BEING THICKEST AT SAID CIRCULAR PLANE SURFACE, SAID GROOVES BEING CONTINUOUS FROM THE INNER PERIPHERAL SURFACE OF THE SECTOR TO SAID CIRCULAR PLANE SURFACE, A BACKING PLATE FOR SAID SECTOR ADAPTED TO BE FIXEDLY SECURED TO SAID CIRCULAR DRIVING PLATE, A WEDGE MEANS ENGAGING ONE PERIPHERAL EDGE OF SAID SECTOR, A RIGID MEANS FOR ENGAGING THE OTHER PERIPHERAL EDGE OF SAID SECTOR, SAID WEDGE MEANS AND RIGID MEANS COOPERATING WHEN SAID SECTOR IS MOUNTED ON SAID CIRCULAR PLATE TO HOLD THE SECTOR TIGHTLY AGAINST ITS BACKING PLATE, SAID MEANS FOR ENGAGING THE INNER PERIPHERY OF THE SECTOR FOR COOPERATING WITH THE SECTOR HAVING AN INWARDLY EXTENDING SURFACE PORTION FORMING A LIP, SAID LIP BEING GROOVED WITH A PLURALITY OF GROOVES WHICH ARE CONTINUOUS WITH THE GROOVES IN THE SECTOR AND WHICH ARE CONTINUOUS ACROSS THE FACE OF THE LIP WHEREBY SAID LIP ACTS AS A CHIP BREAKER. 